1. Field of the Invention
The present invention relates to a quantum well semiconductor laser and, more particularly, to a quantum well semiconductor laser with a window structure free from optical output absorption at an end face portion.
2. Description of the Prior Art
The end face portion of a semiconductor laser absorbs a portion of an optical output and its temperature rises locally. For this reason, oxidation of the crystal progresses at the end face portion during a long-term operation to degrade the semiconductor .laser. In addition, local thermal runaway occurs in the crystal at the end face portion during a high optical output operation to melt the crystal, thereby instantaneously degrading the semiconductor laser. As a method of reducing this optical absorption at the end face portion, there is known a semiconductor laser with a window structure including a semiconductor layer having a larger band gap than the wavelength energy of the optical output at the end face portion.
Various types of semiconductor lasers with a window structure are available. One such structure has a quantum well active layer which is thin only near an end face portion by utilizing the fact that the effective band gap increases with a decrease in thickness of the quantum well layer of a quantum well semiconductor laser. For example, in a semiconductor laser described in Japanese Unexamined Patent Publication No. 4-206982, a projecting mesa structure sided laterally by a terrace structure is formed on a semiconductor substrate by etching beforehand. An active layer is formed on the mesa projection. The active layer is thick at a portion where the terrace structure is closeby and thin at a portion near an end face portion where the terrace structure is absent or distant, thereby forming a window structure.
On the other hand, a selective metal organic chemical vapor deposition (MOCVD) growth method in which parts of the surface of a semiconductor substrate is covered with an SiO.sub.2 film has been attempted. Paper C-131 in the Proceedings of the 1991 IEICE Fall conference describes a method of manufacturing a semiconductor laser in which an InP substrate is covered with a pair of SiO.sub.2 masks, and an InGaAs multiple quantum well layer and an optical waveguide including an InP cladding layer are selectively grown in a stripe region sandwiched by these masks. This paper reports that a smooth (111)B surface is formed on the side surface of a selectively grown portion, and that the selective growth rate is associated with the width of the masks.
A conventional quantum well semiconductor laser with a window structure requires a complicated manufacturing method and is manufactured through a fine etching process, an impurity diffusion process, a crystal growth process requiring strict process management, and the like. The etching process and the diffusion process tend to cause crystal defects, thereby frequently degrading the quantum well semiconductor laser. In the conventional manufacturing method, it is difficult to increase the difference between the effective band gap of an active layer in a window structure portion and the effective band gap of an active layer in an oscillation region. For this reason, relatively large optical absorption occurs at an end face portion, and an end face degradation preventive effect of the window structure is insufficient.
In the semiconductor laser described in Japanese Unexamined Patent Publication No. 4-206982 as an example, the difference between the thicknesses of well layers of active layers in an end face portion and an laser oscillation region is associated with many factors such as the width of a mesa projection, the distance from the mesa projection, the depth of a groove, and the crystal growth rate. These factors, in turn, are also factors influencing formation of an optical waveguide and a current constriction structure, thus presenting contradictory conditions. It is therefore difficult to form an effective window structure. In addition, since an active multilayered film is grown on a substrate crystal with a stepped structure, crystal defects which lead to degradation of the laser tend to form with a high possibility.
In the selective growth method for covering the surface of a semiconductor substrate with a dielectric film, a polycrystalline film inevitably grows on the dielectric film in an Al-containing semiconductor layer of, e.g., AlGaAs or AlGaInP. This method can, therefore, be applied only to a semiconductor laser having a cladding layer not containing Al, such as an InP cladding layer. A semiconductor laser having window structures at two end face portions using this selective growth has not yet been reported.